1. Field of the Invention
The present invention relates to a surface acoustic wave filter including a plurality of edge-reflection type surface acoustic wave resonators connected in a ladder arrangement and more particularly, the present invention relates to a surface acoustic wave filter, duplexer, and communications apparatus having a small size and using significantly fewer internal bonding wires than conventional devices.
2. Description of the Related Art
A surface acoustic wave filter including a plurality of surface acoustic wave resonators connected in a ladder arrangement has been proposed as a band-pass filter to be used in a mobile communications apparatus such as a cellular phone or other such apparatus.
For example, Japanese Unexamined Patent Publication No. 53-123051 discloses a surface acoustic wave filter including a plurality of edge-reflection type surface acoustic wave resonators which make use of a SH type surface wave such as a shear wave, as an example of the above-mentioned surface acoustic wave filter.
FIG. 18 schematically shows one example of such a conventional surface acoustic wave filter. A surface acoustic wave filter 101 includes a plurality of edge-reflection type resonators 102, 103 connected in a ladder arrangement and which makes use of a piezoelectric surface shear wave as a SH type surface wave. As shown in FIG. 18, between input-output ports 104, 105, three edge-reflection type surface-wave resonators 102 are arranged as a series arm and three edge-reflection type surface-wave resonators 103 are arranged as parallel arms.
The edge-reflection type resonators 102 and 103 include piezoelectric substrates 102x and 103x on which IDTs 102y and 103y are disposed, respectively, and each of the edge-reflection type resonators 102 and 103 is connected via bonding wires so as to define a ladder circuit.
The conventional surface acoustic wave filter has the following drawbacks. Specifically, since each IDT 102y and each IDT 103 are disposed on separate piezoelectric substrates 102x and 103x, respectively, it is necessary to electrically connect together the IDTs 102y or connect together the IDTs 103y or interconnect the IDTs 102y and 103y via bonding wires in order to form a ladder circuit. This requires a complicated bonding and connection process.
Further, in edge-reflection type surface acoustic wave resonators, the edge surface for reflecting a SH wave is required to be formed with very high precision in order to function properly. Since the edge surfaces are formed when the piezoelectric substrates 102x, 103x are cut by dicing, the process of dicing is required for each of the piezoelectric substrates. Accordingly, the number of dicing processes is very large and the manufacturing process is also complicated.